p-Toluic acid is an important chemical intermediate, which is widely used to prepare finished products in the manufacture of medicines, agro-chemicals, dyestuffs, optical brighteners etc. It is also used in organic synthesis of various chemical compounds. p-Toluic acid is produced as by- product during the production of tere-phthalic acid/dimethyl tere-phthalate, which are mainly produced by liquid phase oxidation of p-xylene by oxygen/air in acetic acid medium.
Oxidation of p-xylene to tere-phthalic acid in acetic acid solvent in the presence of bromine or bromine containing compound as initiator and a catalyst containing cobalt and manganese components has been disclosed in U.S. Pat. No. 2,833,816 and is practiced world over. Homogeneous catalyst system consisting of cobalt, manganese and bromide is the heart of this process. Oxygen compressed in air is used an oxidant and acetic acid as solvent.
Although the use of bromine is advantageous for such liquid phase oxidation, but its use is associated with some drawbacks. The highly corrosive bromine acetic acid water environment during oxidation require the use of costly titanium lined equipments in some part of the process. Additionally, during reaction bromine also produce methyl bromide, which is hazardous gas. Moreover, in the bromine promoted process, the reaction mixture must be maintained “Substantially anhydrous” (U.S. Pat. No. 3,064,044).
Another patent (U.S. Pat. No. 3,046,305) described a process for the preparation of p-toluic acid starting from toluene, wherein toluene is reacted with chloroformamide under the conditions of Friedel and Craft reaction, and the product thus obtained is hydrolysed to p-toluic acid. This process is not in practice.
There are other patents which report oxidation of p-xylene to terephthalic acid in the presence of water. Patent (U.S. Pat. No. 4,334,086) assigned to Labofina, S. A., Belgium describes a two stage process for oxidation of p-xylene at the 170° C. in the presence of Co—Mn catalyst and 10% (wt %) water in first stage. In second stage partially oxidized compounds are further oxidized at 200° C. in the presence of 20-70% water. After the oxidation, water soluble compounds are separated from insoluble terephthalic acid at 180-200° C., which include p-toluic acid, catalyst and other by products, which are recycled for further oxidation. Crude terephthalic acid crystals obtained in sedimentation column still have about 4.5% p-toluic acid and 2.5% 4-carboxybenzaldehyde as impurities. Labofina. S. A., (U.S. Pat. No. 4,357,475) also described a correlation between temperature of sedimentation column and the oxidation temperature for similar process of oxidation of p-xylene in the presence of water as described above. Labofina. S. A., (U.S. Pat. No. 4,259,522) also described similar process for iosphthalic acid by oxidizing m-xylene in the presence of water. Most of the patents, including above mentioned patents describe the processes for the oxidation of p-xylene to produce tere-phthalic acid as the main product. Little or scanty information is available for the processes to selectively produce p-toluic acid, by liquid phase oxidation of p-xylene in aqueous medium. Studies on oxidation of p-xylene to p-toluic in acetic acid medium in presence of bromide initiator has been described by S. H. Zaidi [Applied Catalysis, 27, 99-106 (1986)]. Therefore, a process to produce p-toluic acid as main product by liquid phase oxidation of p-xylene in water under mild operating condition without using bromine compounds as initiator would not only be of a great economic and commercial potential, but also be an environment friendly technology.